- Structure
- Nomenclature
- Properties
- Physical state
- Solubility
- Characteristics of commercial PACs
- Chemical properties
- Behavior of the PAC in water
- Function of PAC as a flocculant
- Obtaining
- Hydrolysis
- Polymerization
- Importance polymer
- Applications
- - In water treatment
- How does it work
- Advantage
- -In the pulp and paper industry
- - To improve the cement
- References
A class of water-soluble inorganic aluminum products is called poly aluminum chloride, formed by the partial reaction of aluminum chloride AlCl 3 with a base. It is a white to yellow solid. Its general formula is often expressed as Al n (OH) m Cl (3n-m). They are also known as PAC or also PACl (PolyAluminum Chloride).
PACs are formulated in such a way that they contain highly cationic polymers (sets of several molecules with many positive charges) made up of aluminum ions (Al 3+), chloride ions (Cl -), hydroxyl ions (OH) - and water molecules (H 2 O).
Poly Aluminum Chloride (PAC) is used to remove organic and inorganic matter from water in flocculators of wastewater treatment plants. Author: Kubinger. Source: Pixabay.
The most important cationic polymer of these species is called Al 13 or Keggin-Al13 which is very effective in water treatment and in the pulp and paper manufacturing industry.
In these applications, the PACs adhere to the surface of the particles causing them to bind together and can settle, that is, fall to the bottom and can be filtered.
It has also been successfully tested to improve the properties of Portland cement, as it modifies or changes its structure at a micro level and this makes the cement more resistant.
Structure
PAC or PACl is made up of a series of species ranging from monomers (a single molecule), dimers (two molecules joined together), oligomers (three to five molecules joined together) to polymers (many molecules joined together).
Its general formula is Al n (OH) m Cl (3n-m). When these species contain the ions A dissolved in water 3+, hydroxyl ions OH -, chloride ions Cl - and water molecules H 2 O.
In aqueous solution its general formula is Al x (OH) y (H 2 O) n (3x-y) + or also Al x O z (OH) y (H 2 O) n (3x-y-2z) +.
The most useful of these polymers is the one called Al 13 or Keggin-Al13 whose formula is AlO 4 Al 12 (OH) 24 (H 2 O) 12 7+. The species Al 13 has a three-dimensional shape.
It is estimated that the precursor of this polycation is Al (OH) 4 -, which has a tetrahedral conformation and is found in the center of the structure.
Nomenclature
- Aluminum polychloride
- PAC (Poly Aluminum Chloride)
- PACl (Poly Aluminum Chloride)
- Polyaluminum Chloride
- Aluminum polyhydroxychloride
- Aluminum hydrochloride or ACH (Aluminum ChlorHydrate).
Properties
Physical state
White to yellow solid (powder) that is also obtained in the form of aqueous solutions of different concentrations.
Solubility
Soluble in water.
Characteristics of commercial PACs
The various PACs differ from each other mainly by two things:
- Its strength, expressed as% of alumina Al 2 O 3.
- Its basicity, which indicates the amount of polymeric material in the PAC, and can vary between 10% (low basicity), 50% (medium basicity), 70% (high basicity) and 83% (highest basicity, which corresponds to aluminum hydrochloride or ACH).
Chemical properties
PAC is a kind of water soluble aluminum products. Its general formula is often expressed as Al n (OH) m Cl (3n-m).
Since they are produced by reacting aluminum chloride (AlCl 3) with a base, the basicity of these types of products depends on the relative amount of OH - ions compared to the amount of aluminum (Al).
According to the formula Al n (OH) m Cl (3n-m), the basicity is defined as m / 3n.
It is a flocculant. It has properties such as ease of adsorption on other particles of opposite charge (it adheres to the surface of these), coagulation (union of several particles on which it has been adsorbed) and precipitation of these groups of united particles.
PACs can be unstable as they depend on pH. They can be corrosive.
Behavior of the PAC in water
By dissolving the PAC in water and depending on the pH, various aluminum-hydroxyl (Al-OH) species are formed.
It hydrolyzes or reacts with water to form monomers (unitary molecules), oligomers (3 to 6 molecules linked) and polymers (more than 6 linked molecules).
The most important species is a polymer with 13 aluminum atoms, which is called Keggin-Al13.
Function of PAC as a flocculant
The Keggin-Al13 polymer adsorbs on the particles present in the water, that is, it sticks on the surface of these, and causes them to add to each other, forming flocs.
Flocs are groups of very small particles agglutinated or united to form larger structures that can sediment, that is, go to the bottom of the aqueous solution.
After forming flocs, when they are large enough they go to the bottom and the aqueous solution is clean.
Flocculation and sedimentation tanks of a water treatment plant where poly aluminum chloride (PAC) can be used. Qualit-E at English Wikipedia. Source: Wikimedia Commons.
Obtaining
PAC or PACl solutions are generally obtained by adding a base or alkaline solution to a solution of aluminum chloride (AlCl 3).
For a high amount of Al 13 polymers to be obtained, the base or alkali that is added must not provide OH ions - too fast or too slow.
Some studies indicate that it is difficult to produce a stable high concentration of Al 13 using NaOH because it releases OH ions - too fast into the water.
For this reason basic calcium (Ca) compounds are preferred, which have low solubility in water and thus release OH ions - slowly. One of these basic calcium compounds is calcium oxide CaO.
Here are the steps that occur for the formation of the PAC.
Hydrolysis
When aluminum salts (iii) dissolve in water, a spontaneous hydrolysis reaction occurs in which the aluminum Al 3+ cation takes hydroxyl ions OH - from water and binds to them, leaving free H + protons:
Al 3+ + H 2 O → Al (OH) 2+ + H +
Al 3+ + 2 H 2 O → Al (OH) 2 + + 2 H +
This is aided by adding an alkali, that is, OH - ions. The aluminum ion Al 3+ is increasingly joining OH - anions:
Al 3+ → Al (OH) 2+ → Al (OH) 2 + → Al (OH) 3 0 → Al (OH) 4 -
In addition, species such as Al (H 2 O) 6 3+ are formed, that is, an aluminum ion bound or coordinated with six water molecules.
Polymerization
Then bonds are formed between these species, forming dimers (sets of 2 molecules) and trimers (sets of 3 molecules) that are transformed into oligomers (sets of 3 to 5 molecules) and polymers (sets of many joined molecules).
Al (OH) 2 + → Al 2 (OH) 2 4+ → Al 3 (OH) 5 4+ → Al 6 (OH) 12 6+ → Al 13 (OH) 32 7+
This type of species is linked by OH bridges with each other and with Al (H 2 O) 6 3+, forming sets of molecules that are called hydroxy complexes or polycations or hydroxypolymers.
The general formula of these cationic polymers is Al x (OH) y (H 2 O) n (3x-y) + or also Al x O z (OH) y (H 2 O) n (3x-y-2z) +.
Importance polymer
The most useful of these polymers is thought to be the so-called Al 13 whose formula is AlO 4 Al 12 (OH) 24 (H 2 O) 12 7+, and it is also known as Keggin-Al13.
It is a species with 7 positive charges (that is, a heptavalent cation) with 13 aluminum atoms, 24 OH units, 4 oxygen atoms, and 12 water H 2 O units.
Applications
- In water treatment
PACl is a commercial product to treat water and make it drinkable (clean and drinkable). It also allows treating waste and industrial waters.
The water can be made drinkable if it is treated with poly aluminum chloride (PAC). Author: ExplorerBob. Source: Pixabay.
It is used as a coagulation agent in water improvement processes. It is more effective than aluminum sulfate. Its performance or behavior depends on the species present, which depends on the pH.
How does it work
PACl allows organic material and mineral particles to be coagulated. Coagulate means that the compounds to be eliminated go from being dissolved to being solid. This is achieved through the interactions of its positive charges with the negative ones of the materials to be coagulated.
It is believed that the Al 13 species, having so many positive charges (+7), is the most effective in neutralizing charges. Then there is the formation of bridges between the particles which agglomerate and form flocs.
These flocs, being very heavy, tend to precipitate or settle, that is, to go to the bottom of the container that contains the water that is being treated. In this way they can be removed by filtration.
Poly aluminum chloride (PAC) is used to sediment organic and inorganic matter in wastewater treatment plants. US Army Corps of Engineers photo. Source: Wikimedia Commons.
Advantage
PAC is better than aluminum sulfate because it has better low temperature performance, leaves less aluminum residue, produces less sludge volume, less effect on water pH, and faster and larger flocs are formed. All of which facilitates sedimentation for subsequent filtration.
Pool water can be purified with poly aluminum chloride (PAC). Author: Kalhh. Source: Pixabay.
-In the pulp and paper industry
PAC is especially effective in modifying colloidal fillers in papermaking. Colloidal charges are the charges of the suspended solids in the mixtures to make paper pulp.
It allows to accelerate the speed of drainage (elimination of water) especially in neutral and alkaline conditions, and helps in the retention of solids. The solids are those that later, when drying, form the paper.
In this application PAC with low (0-17%) and medium (17-50%) basicities are used.
In pulp and paper mills, poly aluminum chloride (PAC) is used to aid in the sedimentation process. Author: 151390. Source: Pixabay.
- To improve the cement
Recently (2019) adding PACl to Portland cement has been tested. It was determined that the presence of Cl - chloride ions and of polymeric aluminum groups change the structure of the cement. It is estimated that complex salts of the formula 3CaO.Al 2 O 3.CaCl 2.10H 2 O are formed.
Construction cement can be enhanced with poly aluminum chloride (PAC). Skeeze. Source: Wikimedia Commons.
The results indicate that PACl improves the properties of the cement, decreases the number of micropores (very small holes) and the matrix becomes more dense and compact, therefore the resistance to compression increases.
The effect increases with increasing content of PACl. The study confirms that adding PACl to Portland cement produces a mixture with superior mechanical and microstructural properties.
With poly aluminum chloride, the porosity of the cement decreases and it becomes more resistant. Blackblack111. Source: Wikimedia Commons.
References
- Kim, T. et al. (2019). Investigating the Effects of Polyaluminum Chloride on the Properties of Ordinary Portland Cement. Materials 2019, 12, 3290. Recovered from mdpi.com.
- Li, Y. et al. (2019). Optimization of Polyaluminum Chloride-Chitosan Flocculant for Treating Pig Biogas Slurry Using the Box-Behnken Response Surface Method. Int. J. Environ. Res. Public Health 2019, 16, 996. Recovered from mdpi.com.
- Hubbe, M. Polyaluminum Chloride (PAC). Mini-Encyclopedia of Papermaking Wet-End Chemistry. Recovered from projects.ncsu.edu.
- Tang, H. et al. (2015). Speciation, stability, and coagulation mechanisms of hydroxyl aluminum clusters formed by PACl and alum: A critical review. Adv Colloid Interface Sci 2015; 226 (Pt A): 78-85. Recovered from ncbi.nlm.nih.gov.
- Bottero, JY et al. (1980). Studies of Hydrolized Aluminum Chloride Solutions. 1. Nature of Aluminum Species and Composition of Aqueous Solutions. The Journal of Physical Chemistry, Vol. 84, No. 22, 1980. Retrieved from pubs.acs.org.
- Zhao, H.-Z. et al. (2009). High-concentration polyaluminum chloride: Preparation and effects of the Al concentration on the distribution and transformation of Al species. Chemical Engineering Journal 155 (2009) 528-533. Recovered from sciencedirect.com.
- Jia, Z. et al. (2004). Synthesis of Polyaluminum Chloride with a Membrane Reactor: Operating Parameter Effects and Reaction Pathways. Ind. Eng. Chem. Res. 2004, 43, 12-17. Recovered from pubs.acs.org.
- GEO Specialty Chemicals. Polyaluminum Chloride (PAC). Recovered from geosc.com.